Objective of the proposed research is to elucidate the biochemical mechanisms involved in active cation transport by the normal kidney, and in certain pathologic conditions characterized by altered renal handling of sodium and potassium. To obtain a better understanding of the relationship between Na-K-ATPase and other transport ATPases on the one hand and renal function on the other we have developed micromethods for the determination of these enzymes in single nephron segments. These techniques make it possible to measure the enzymes in tubule fragments in which the spatial orientation of the cells is preserved, and at the same time allow the precise localization of the function under study. We will evaluate the activity, properties and physiologic role of a Ca-dependent ATPase present in the cell membrane and presumably involved in the transcellular transport of calcium. This enzyme will be studied in microdissected single tubules as well as in tissue preparations enriched in basolateral cell membranes obtained from kidney cortex homogenates.